1. Field of the Invention
The present invention relates to a GPS terminal and position measuring system which measure the current position of an object by using a GPS (Global Positioning System) and display a map centered on the measured position, and a map display method using them and, more particularly, to a GPS terminal and position measuring system which can automatically change the sale of a map to be displayed, and a map display method using them.
2. Description of the Prior Art
As a system for measuring the current position of a moving object such as a vehicle, a position measuring system called a GPS is widely used, in which a moving object receives radio signals from a plurality of (three or more in general) artificial satellites and measures the current position of the moving object by using the received radio signals.
In a position measuring system using a GPS, map data is stored in a GPS-equipped GPS terminal in advance, and a map centered on the measured position is displayed on the screen of the GPS terminal, thereby allowing the user recognize the current position of the GPS terminal.
Of position measuring systems using GPSs, a network-dependent position measuring system in which each GPS terminal is equipped with a network-dependent GPS to measure the current position of the GPS terminal by using network communication has become popular, as compared with a conventional position measuring system in which each GPS terminal is used singly.
In comparison with a general GPS terminal using a GPS singly, a GPS terminal equipped with a network-dependent GPS exhibits high reception sensitivity with respect to radio signals from artificial satellites and a short communication establishment time for artificial satellites because of accurate calculation of position information on the network side.
According to such a GPS terminal equipped with a network-dependent GPS, when the reception sensitivity level with respect to radio signals from artificial satellites is high, the position of the GPS terminal can be recognized with a precision of several m. When, however, the reception sensitivity level is low, e.g., the GPS terminal is influenced by a multi-path effect between buildings, in a room, or the like, or the network is disconnected, the recognition precision of the position of the GPS terminal deteriorates to several hundred m.
In a general GPS terminal using a GPS singly, while the reception sensitivity level is low, the terminal cannot establish communication with any artificial satellite before deterioration in position recognition precision., and hence the position cannot be quickly recognized.
If, therefore, the reception sensitivity level with respect to radio signals from artificial satellites drops and the position recognition precision deteriorates while a map is displayed on the screen at a small reduction ratio, an accurate map may not be displayed on the screen. Then, the user of the GPS terminal may erroneously recognize the current position more often.